This invention relates to a method of producing polyethylene or ethylene copolymer open-cell foam materials which are high-expansive or ultra-high-expansive materials with an expansion ratio of at least 25 times, and which have an open-cell ratio from 96 to 100%.
A conventional method dealing with the production of polyethylene or ethylene copolymer open-cell foam materials is described in Japanese Patent Publication No. SHO. 59-23545 (hereafter referred to as the conventional method). This conventional method includes three process steps: a first step in which a compound containing a foaming agent and a cross-linking agent is heated for a fixed period of time, thus forming a bridged first intermediate product in which the decomposition ratio of the foaming agent is 30% or less; a second step in which the remaining foaming agent and cross-linking agent are decomposed to form a second intermediate product having easily ruptured closed cells; and a third step in which the second intermediate product is compressed within a temperature range of about 0.degree. to about 40.degree. C. in order to rupture the closed cells.
The following three problems (a to c below) are inherent in the conventional method.
(a) Judging from the embodiment examples of the conventional method, the expansion ratio of the second intermediate product is at a maximum of 6 to 15 times. (Although it is stated in the specification that an expansion ratio of 3 to 25 times is appropriate for the second intermediate product, even when all of the embodiments are taken into consideration and additional tests are performed, the maximum expansion ratio for the second intermediate product cannot be considered to be more than 15 times). PA1 (b) Thus, for the expansion ratio of the final product obtained through the compression rupturing of the second intermediate product, because the closed cells are simply ruptured by compression, it is essentially impossible to obtain an expansion ratio of greater than 15 times. In other words, the conventional method is not capable of providing so-called high expansive or ultra-high-expansive materials with an expansion ratio of 25 times or more. PA1 (c) According to the information in the specification of the above Japanese patent publication, the final product obtained using the conventional method has an open cell ratio of no more than 95%. In other words, because the final product is an open-cell foam material containing at least 5% independent cells, it lacks compression recovery, has poor water absorption and permeability, and is incapable of providing sufficient performance as an open-cell foam material. In short, the conventional method does not even come close to providing a product with a 100% open-cell ratio.
One may draw the following conclusions from a study of the reasons behind the problems described above. Because the decomposition ratio of the foaming agent in the first step is a maximum of 30% (the foaming agent decomposition ratio is listed in the specification as being preferably no more than 15%, and ratios of 12 to 25% are listed for the embodiments), even if the foaming agent decomposition ratio in the first step is low (foaming agent decomposition ratio of 15% or less), the decomposition of the foaming agent can also be considered to be a necessary condition. The fact that the decomposition of the foaming agent is a necessary condition even though the foaming agent decomposition ratio is no more than 30% means that the foaming agent must be rapidly decomposed in the foaming which occurs in the next process, and this necessarily requires that the foaming agent be processed beforehand (the heating to temperatures of 135.degree. to 143.degree. C. listed in the specification) so that it will be easily decomposed, and this heat processing can be thought to be designed to cope with the fact that the foaming agent will not decompose more than 30%. Thus, the range of the heating temperature in the first process can be thought to have a vital influence on the decomposition of the foaming agent.
In addition, because the decomposition of the foaming agent in the first step is done under pressure, the cell membranes which are formed at this time are as a result generally strong, and the thickness of the cell membranes remains unchanged through the completion of the second step, so that they are not ruptured by the mechanical compression in the final process. In effect, this can be considered to be one reason for the existence of closed cells.
Furthermore, according to the specification, the conditions for the heat processing in the second step of the first intermediate product obtained from the first step are temperatures of 190.degree. to 210.degree. C. However, as stated in the specification, the thicknesses of the cell membranes formed by the decomposition of the remaining undercomposed foaming agent are not uniform. Thus, when mechanical compression is used in the final step to rupture the cells, although the cells rupture at places where the cell walls are thin, they tend not to rupture at places where the cell walls are thicker. The result is that it is impossible to obtain any kind of open-cell foam material other than one which partially contains closed cells.
Based on the conclusions described above concerning the prior art method, and on the results of many experiments and much consideration, the present inventor has arrived at the following conclusions.
Ideally, it is best if the foaming agent does not decompose at all (foaming agent decomposition ratio of 0%) during the first step, and even if there is some decomposition, it should be kept extremely low (within 4%). This can be accomplished by meeting at least two conditions: (1) keeping the temperature for heating in the first step below 135.degree. C., more specifically to within 110.degree. to 135.degree. C., and (2) keeping the amount of foaming assist agent added to within 0 to 0.45 parts by weight. It was confirmed by actual experiment that satisfying both of these conditions makes it possible to keep the decomposition of the foaming agent during the first step to within 0 to 4% (at an expansion ratio of 25 times).
In addition, in order to achieve an open-cell foam material in the third step which does not partially contain closed cells, the thickness of the cell walls in the second step must be approximately uniform and, moreover, thin. In order to accomplish this, the decomposition of the foaming agent should be delayed and the foaming action controlled to occur slowly. By doing this, it is possible to form closed cells with cell membranes which are both uniform in thickness and thin. It was confirmed by actual experiment that, in order to accomplish this, it is important to set the temperature for non-pressurized heating to 150.degree. to 190.degree. C., which is lower than that used in the prior art method.
This invention is based on the experimentally confirmed facts described above, and the purpose of the invention is to provide a method for the production of polyethylene or ethylene copolymer open-cell foam materials in which the final product has an ultra-high expansion ratio of 25 times or more and an open cell ratio of virtually 100%.